This series is very much a character study, and a dark one at that. It's about a teen (Tsukasa) who seeks refuge in a very immersive MMORPG because of an abusive father, before falling into a catatonic state, becoming trapped in a fantasy world called, well, The World. With the help of people met on the way, Tsukasa slowly discovers why the entrapment happened in the first place, while changing as a person.
The crux of the plot is the mystery of it all, as the virtual world is pretty much a black box that got appropriated for use as a game without a thorough investigation of its hidden, sinister elements that come to the forefront as the story progresses.
However, what's great about this series isn't the plot, but the characters and the themes. The most prominent theme is escapism, and how we can run away from the trying circumstances of our lives by immersing ourselves within an engaging fictional world. The real world is portrayed as dull and bleak, potently represented in monochrome and silence as everyone's eyes are obscured. There's also a very subtle and well-done romance between Tsukasa and Subaru, which never feels forced or overly sentimental, but instead feels natural and makes for a very touching and positive ending, which helps to clench the whole series for me.
That ending: eventually, Tsukasa is able to log out of The World with the help of people met there, and once out, decides to meet up with Subaru in the real world. It's revealed that Tsukasa is a girl, and that Subaru is confined to a wheelchair. Are these elements played up? Commendably, no, and the fact that Tsukasa and Subaru are a same-sex couple is also not made an issue (but all of these elements enhance the narrative and is perfectly consistent with what we know about the characters and their motivations). Instead, the ending shows their touching reunion in the real world, as the bleak presentation is relaxed somewhat, providing a perfect conclusion to the themes of the story that tells us this: even if the real world can give us tragedies, and has imperfections, the answer isn't to completely escape from it, but to find happiness in our own lives in whatever way we can.
This review didn't even touch on the unique beauty of The World, the other great characters or the atmospheric music. But it didn't need to. How the themes are explored is compelling enough.